1. Field of the Invention
This invention relates to floating fasteners and, more particularly, to the design, manufacture and assembly of a floating fastener having an integrally-formed nutplate and retention clip that provides the same reliable fastener installation at far lower cost.
2. Description of the Related Art
In many structural applications, structural members need to be fastened together. Oftentimes the structural members are too thin, too soft or otherwise too fragile to simply drive a screw through the members to form a reliable joint. Furthermore, misalignment of the structural members will exert a side loading on the screw that will limit the strength of joint. A common solution is to form aligned axial through-holes in the structural members having a diameter greater than that of the screw threads. A threaded nut is held on one side and the screw is driven through the axial through-holes into the nut so that the screw is placed under tension with no side loading to form a strong and reliable bolted joint at the interface of the two members.
To support cost-effective manufacturing and assembly, the axial through-hole on the interior structural member is oversized, which relaxes the positional tolerance on manufacturing the holes and assembling the structural device to align the axial through-holes. In many applications, there may be dozens of through-hole pairs that need to be simultaneously aligned and then fastened. To further complicate matters, in situations referred to as ‘blind access’ the machine or technician that is installing the screw does not have access to the backside of the assembly to hold the nut. In these cases, a ‘floating fastener’ is pre-assembled on the backside of the interior structural member. The floating fastener includes the threaded nut and a nutplate that captures the nut but allows it to ‘float’ i.e. move around freely inside the nutplate, to accommodate misalignment of the axial through-holes within a designed for tolerance. The lead chamfer on the screw will engage the nut and move it over so that the nut and screw are properly aligned.
Due in large part to the inability to access the backside of the structure once assembly has begun, the floating fastener assemblies must be highly reliable; they must work every time. Rework is slow and expensive. The floating fastener must have a low risk of installation damage e.g., damage to the structural members and particularly the axial through-holes, and must have a low risk to installed performance e.g. the nut won't fall off prior to assembly and the nutplate will provide the requisite axial and torque resistance to hold the nut in place to install the screw properly. Without sacrificing reliability, the “per hole” cost of each fastener including components and labor should be as low as possible. Structural applications may require dozens of floating fasteners and the costs add up quickly.
As illustrated in FIG. 1, existing designs of a floating fastener 10 that are currently available including “riveted”, which is the MIL-spec standard, “click bond” and “press fit” place a nut 12 (e.g. MIL-spec NAS 1794) including a threaded barrel 14 on a base 16 in a discrete nutplate 18. The blank nutplate 18 is machined with four protrusions 20 at its corners that are crimped to capture the base 16 of nut 12 yet allow it to float within the cage. The floating fastener is secured by some ‘means’ to the backside of an interior structural member 22 so that the barrel lies within the extent of an oversized axial through-hole 24. An exterior structural member 26 having axial through-hole 28 is placed over the interior structural member 22 so that their through-holes 28 and 24 are roughly aligned providing enough overlap to insert a screw 30 so that its lead chamfer 32 will engage the barrel and move the nut over so that the nut and screw are properly aligned. The cost associated with machining of the nutplate 18 and the labor to crimp the nutplate to capture the nut is not insignificant, approximately $0.90 to $2.40 per hole plus labor. All of the existing designs use some singular means' to align the nutplate and nut to the axial through-hole and hold it in place, to provide the axial resistance required so that the lead chamfer engages the nut and resists the axial load from driving the screw into the nut, and to provide the torque resistance required to prevent the nut and nutplate from turning. The different means vary in reliability and cost.
As shown in FIG. 2a, a riveted floating-fastener 40 includes a nut 42 and nutplate 44 in which the nutplate is machined with a pair of flanges 46 on opposite sides of the nut each having a precision hole 48 formed therein for receiving a rivet. In an alternate embodiment, for use in corners for example, both precision holes 48 are formed in a single flange to one side of the nut. The precision holes 48 must be aligned and spaced to precisely match complementary holes formed on opposite sides of the axial through-hole in the interior structural member. Rivets are driven through precision holes 48 into the mating holes in the interior member. Each rivet is a compressed metal column that expands outward to fill the hole in the structural member to hold the nutplate in place. The riveted floating-fastener is the mil-spec standard because of the low technical risk associated with the rivets, they provide a very strong and reliable joint to hold the nutplate. There is some risk of installation damage during rivet installation. The tradeoff is that the assembled cost is quite high, approximately $6.69 per hole for Raytheon's JSOW. The riveted fastener requires five machining operations using three different tools and three different assembly operations.
As shown in FIG. 2b, a click-bond floating fastener 50 includes a nut 52 and a nutplate 54 that is adhesively bonded to the backside of the interior structural member to position nut 52 below the axial through-hole in the member. An applicator 56 is inserted through the nut and used to align the nut and nutplate to the axial through-hole and bond the nutplate using a two-part adhesive and is then removed. Pushing on the applicator during assembly poses some risk. This approach requires only a single machining operation and a single assembly operation but uses two-part adhesives that require 7 days to cure. This fastener uses the same mil-spec nut and a modified nutplate without flanges. The total cost is lower than the riveted fastener, approximately $4.08 per hole, but still high. However the adhesive bond is not as reliable as the rivets. The bond is known to fail occasionally under the axial and/or torque loading when installing the screw. The bond can also be stressed through a difference in thermal expansion at temperature extremes.
As shown in FIG. 2c, a press-fit floating fastener 60 includes a nut 62, a sleeve 64 formed with teeth around is circumference and a nutplate 66. The sleeve is mounted on a mandrel 68 that pulls the sleeve through the axial through-hole in the interior structural member. The teeth deform the metal on the inside of the hole to hold the nutplate in place and provide axial and torque resistance. Once press-fit into the hole the mandrel is removed. This approach requires only a single machining operation and a single assembly operation using a special tool. The total cost is similar to the adhesive bonding, approximately $4.12 per hole. The installation risk is also similar to bonding in that pressing the teeth into the hole may cause damage. The bigger problem is that the deformation of the axial through-hole creates ‘stress risers’ that weaken the interior structural member and reduce the reliability of the joint.
The industry has an unfulfilled need for a ‘floating-fastener’ that provides the same reliability as the riveted fastener but at a much lower total cost per hole. Preferably any such solution could use the MIL-spec nut currently accepted by the industry.